Heat the olive oil in a large cast iron pan over medium heat. Add the Brussel sprouts, cooking until lightly browned and vibrant green. Add the bacon, continuing to cook for 2 minutes. Remove to a bowl and return hot pan to the stove.
Add the butter, shallot and garlic to the hot pan over low heat. Saute until slightly soft, approximately 2 minutes.
Stir in the flour until it forms a paste, then stir in the cream and bring to a low dimmer. Stir in the Dijon mustard, salt and pepper. Mixture will be thick like gravy. Remove from the heat.
Stir back in the Brussels sprouts and bacon until coated. Top with shredded mozzarella and parmesan cheeses.
Bake in the oven for 10 minutes, turn on the broiler and brown for 1-2 minutes, keeping a watchful eye. Top with more salt and pepper, if desired.
Serve immediately.
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